1. Field of the Invention
The present invention is directed to a local area network (LAN) and, more particularly to a LAN having a star topology, but using a ring protocol to connect each active node to two adjacent active nodes, one transmitting data to the node and the other receiving data from the node.
2. Description of the Related Art
There are three primary network topologies used in local area networks (LANs), a linear bus, a star and a ring. For purposes of speed and security, it is desirable to connect nodes in a LAN using optical fibers. Due to technical problems with optical taps, a linear bus topology is difficult to implement using optical fibers.
A star topology is somewhat easier to implement using optical fibers, but it also has drawbacks. In a star coupled network, each of the nodes are connected to a central point via optical fibers and during an assigned time period broadcasts data to all of the other nodes. In a passive optical star coupler, data is broadcast by splitting the optical signal. This results in practical limitations on the size of a passive star coupled network to about 16 nodes due to the reduction in optical power in the splitting process. An active star coupler which regenerates the optical signal within the star can be used for larger networks, but are significantly more expensive and require additional controls that are typically provided by a linear bus and protocol used in conjunction.
A ring topology, using protocols such as the SAE High Speed Ring Bus, ANSI FDDI or IEEE 802.5, is the most easily implemented topology when using optical fibers. Each node typically converts an optical signal into an electrical signal to determine whether data is addressed to that node while regenerating and repeating all signals to the downstream node. As a result, the drawbacks of linear and star topologies with respect to optical implementations are not present in a ring network. However, failure at a signal node can prevent communication. Counter-rotating rings which reverse the direction of data transmission in the ring are one technique for overcoming a single link failure. However, loss of power or failure of a station in the network may be overcome by bypassing the station.
There are two primary optical bypass devices presently in use, a passive optical bypass and an electro-mechanical switch. The passive bypass has a 10 dB insertion loss. Thus, two consecutive passive bypasses would result in 20 dB attenuation which approaches the limit of currently available detectors' ability to receive the optical signal. An electro-mechanical switch uses a solenoid actuator to change the alignment of the optical fiber. This type of bypass typically has a 3 dB insertion loss, switching speed of about 10 ms and is often temperature and vibration sensitive. Thus, no more than about 6 consecutive inactive stations can be present in a ring network with electro-mechanical bypass switches and the network would be more susceptible to environmental factors.